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Abstract

Engineering resonances in metamaterials has been so far the main way of reaching simultaneously negative permittivity and negative permeability leading to negative index materials. In this paper, we present an experimental and numerical analysis of the infrared response of metamaterials made of continuous nanowires and split ring resonators (SRR) deposited on low-doped silicon when the geometry of the SRRs is gradually altered. The impact of the geometric transformation of the SRRs on the spectra of the composite metamaterial is measured in the 20–200 THz frequency range (i.e., in the 1.5–15µm wavelength range) for the two field polarizations under normal to plane propagation. We show experimentally and numerically that tuning the SRRs towards elementary cut wires translates in a predictable manner the frequency response of the artificial material. We also analyze coupling effects between the SRRs and the continuous nanowires for different spacings between them. The results of our study are expected to provide useful guidelines for the design of negative index metamaterials on silicon.

Figures (4)

Schematic representations (top) and scanning electron microscope images (middle and bottom) of the four metamaterial structures fabricated on silicon. The top and bottom pictures show the elementary unit cell of each structure. From left to right, the length of SRR legs is reduced to the point where the SRR is transformed into a single wire piece. The middle pictures show the regularity achieved in the fabrication of the periodic arrays of nanowires and split ring resonators

Measured and simulated transmission/reflection spectra of the four structures depicted in Fig. 1. Curves in red, blue, green and black are for the 1st, 2nd, 3rd and 4th structures, respectively. - (a) and (b): transmission and reflection spectra measured for the parallel polarization (incident electric field parallel to the SRR gap). - (c) and (d): numerical simulations corresponding to (a) and (b), respectively. - (e) and (f): transmission and reflection spectra measured for the perpendicular polarization (incident electric field perpendicular to the SRR gap). - (g) and (h): numerical simulations corresponding to (e) and (f), respectively.

Magnitude of the normal electric field component (|Ez|) calculated at the bottom surface of the metallic elements for each of the resonant plasmon modes observed in the different spectra of Fig. 2. The different colours, from blue to red correspond to increasing magnitudes of the field component. Modes are classified according to the resonance order and to the polarization of the incident electric field. As expected, both the energy and the number of field nodes increase with the resonance order.

(a). Transmission spectra calculated for a periodic array of SRRs (red curve), a periodic array of continuous wires (pink dashed curve) and periodic arrays of SRRs and wires with different separations between SRRs and wires: d=130 nm (blue curve), d=50 nm (green dashed curve), d=10 nm (black dashed curve). In each case, the incident electric field is polarized parallel to the gap, the SRR dimensions and lattice period as the same as for structure 1 in Fig. 1(b): Magnitude of the normal electric field component (|Ez|) calculated at the bottom surface of the metallic elements for each of the resonant modes observed in the transmission spectra of Fig. 4(a). The different colours, from blue to red correspond to increasing magnitudes of the field component. Modes are classified according to the resonance order and to the separation d between the SRR and the closest wire.